Kurdish peshmerga troops participate in an intensive security deployment against Islamic State militants in Makhmur, on the outskirts of the province of Nineveh August 7, 2014 Photograph: Reuters

American warplanes began bombing Islamic militant targets outside the Kurdish city of Irbil on Friday, in the first offensive action by the US in Iraq since it withdrew ground troops in 2011.

Following authority granted by Barack Obama on Thursday, the Pentagon said two FA-18 jets dropped 500-pound laser-guided bombs on fighters with the Islamic State, also known as Isis or Isil.

The US claimed the militants were using artillery to shell peshmerga forces defending Irbil and threatening US personnel in the city.

“As the president made clear, the United States military will continue to take direct action against Isil when they threaten our personnel and facilities,” said Pentagon spokesman Rear Admiral John Kirby.

Rear Adm. John Kirby (@PentagonPresSec)

US military aircraft conduct strike on ISIL artillery. Artillery was used against Kurdish forces defending Erbil, near US personnel.

Obama’s orders to his military commanders were widely drafted and included permission to take action against Isis forces threatening either the thousands of Yazidi refugees trapped on Mount Sinjar, or the cities of Irbil and Baghdad, where US “military advisers” are based.

The US portrayed its initial action on Friday as a necessary step to protect its joint operation centre in Irbil, which is being used to co-ordinate defences with Peshmerga fighters. “The fact of the matter is we have people in Irbil and if Irbil is allow to fall, they will be at risk,” said deputy national security adviser for strategic communications Ben Rhodes on Friday.

But US jets have been operating over Kurdish areas for some time and the Pentagon believes the Islamic militants advancing toward Irbil pose a significant threat to the city.

Since Obama spoke from the White House on Thursday night, there has been relatively little criticism of his return to Iraqi military interventions in Congress. However there is thought to be deep unease within the White House about the risk of being sucked back into a prolonged campaign against Isis.

White House officials, briefing reporters on Thursday night, made clear that the latest intervention was triggered by the rapid Isis advance on Irbil, although Obama also stressed the need for the US to take action to prevent possible genocide of the Yazidi population.

Separate humanitarian air drops have already began to bring relief to the thousands of trapped Yazidis. Obama described the threats against the stranded refugees as holding the potential for “genocide”.

The two FA-18 Hornets that carried out Friday’s operation were launched from the USS George HW Bush aircraft carrier in the Gulf. The operation began at 1.45pm local time (6.45am ET, 11.45am BST).

Following Obama’s authorisation to his military commanders, they are able to act independently of the White House on tactical matters. “The decision to strike was made by the US central command commander under authorisation granted him by the commander-in-chief,” said Kirby.

In Congress, Republicans and many Democrats welcomed the US intervention but questioned White House strategy for dealing the broader threat from Isis.

“The president’s authorization of airstrikes is appropriate, but like many Americans, I am dismayed by the ongoing absence of a strategy for countering the grave threat Isis poses to the region,” said House speaker John Boehner in a statement.

“Vital national interests are at stake, yet the White House has remained disengaged despite warnings from Iraqi leaders, Congress, and even members of its own administration. Such parochial thinking only emboldens the enemy and squanders the sacrifices Americans have made.”

Meanwhile, the Associated Press reported that cargo planes dropped parachuted crates of food and water over an area in the mountains outside Sinjar, where thousands of members of the Yazidi minority where sheltering, according to witnesses in the militant-held town, who asked not to be identified for security reasons.

The Iraqi ministry of immigration and the displaced welcomed the aid drops. The ministry’s spokesman, Satar Nawrouz, said the drops came “just in time.”

Tens of thousands of Yazidis have been trapped in the mountains since the Islamic State overran Sinjar. The militants issued an ultimatum to the Yazidis, telling them they must convert to Islam, pay a religious tax, flee or face death.

Faced with the threats, about 50,000 Yazidis half of them children, according to UN figures fled to the nearby mountains, where they were running out of food and water.

Graphic: Guardian/Paddy Allen

On a visit to India, US defence secretary Chuck Hagel told reporters that more than 60 of the 72 bundles of food and water airdropped onto the mountain reached the people stranded there.

A representative of Iraq’s most influential Shiite cleric, Grand Ayatollah Ali al-Sistani, called for a more comprehensive international intervention to support the Iraqi government.

“The condemnation and consolation statements in support of the affected people, or sending some humanitarian aid, is not enough. Rather, solid plans, in cooperation with the Iraqi government, should be put in place to confront and eliminate the terrorists,” said al-Sistani’s spokesman Abdul-Mahdi al-Karbalaie in his Friday sermon in the holy city of Karbala.

Isis, which has carved out a self-styled caliphate across wide swaths of Syria and Iraq, seized Iraq’s largest hydroelectric dam on Thursday, taking control of enormous power and water resources and leverage over the Tigris River that runs through the heart of Baghdad.

On Thursday the group said it has seized 17 Iraqi cities, towns and targets including the dam and a military base over the past five days, including Qaraqoush, the largest Christian village in Iraq.

In Irbil, about 3,000 Christians who fled their homes in Qaraqoush huddled inside St. Joseph’s cathedral. They said they were happy about the possibility of American airstrikes. “We are pleased with the airstrikes and we hope we can go back to our properties,” said one of the Qaraqoush refugees, 43-year-old Luay Janan.